Since lithium secondary batteries spotlighted recently as power sources for compact electronic instruments use an organic electrolyte, they show a discharge voltage at least 2 times higher than that of the conventional battery using an aqueous alkaline solution, and thus show high energy density.
The conventional lithium secondary battery uses, as a negative electrode active material, a carbonaceous material capable of reversible lithium ion intercalation/deintercalation, while maintaining structural and electrical properties as a negative electrode active material. However, as portable compact electronic instruments have been functionalized diversely and have undergone downsizing and weight lightening, it is required for lithium secondary batteries to have high capacity. Therefore, many attentions have been given to a graphite-based negative electrode material having a higher theoretical capacity than that of graphite (372 mAh/g) used conventionally as a negative electrode material for a lithium secondary battery.
As a negative electrode material other than a carbonaceous material, there is a silicon-based metallic material. Such a silicon-based metallic material is an active material having a theoretical capacity at least 10 times higher than that of graphite, and thus active studies about the material have been conducted. However, such a silicon-based metallic material has not been commercialized yet, because there are problems in that volumetric swelling of silicon particles and a volumetric change occurring during a charge cycle causes cracking, resulting in degradation of conductivity between active material particles, separation of an active material from an electrode plate and continuous reaction with an electrolyte, and thus degradation of life characteristics of a lithium secondary battery.
In addition, there have been increasing studies about lithium metal oxides, particularly a lithium titanium oxide (LTO), having a high charging rate. It is known that a lithium metal oxide has a small particle size and a large specific surface area to allow high rate charge/discharge, shows an excessively low structural change during charge/discharge and zero-strain, provides excellent life characteristics, forms a relatively high voltage range, causes no formation of dendrites, and thus has excellent safety and stability.
However, since a lithium metal oxide shows lower electroconductivity and capacity as compared to a carbonaceous material and has a non-uniform particle shape, it is not mixed homogeneously with a binder and conductive material to be mixed together during the manufacture of a negative electrode.
Therefore, when the content of a binder is increased in order to increase the adhesion, the content of a conductive material or that of an active material is decreased relatively, resulting in degradation of electroconductivity and capacity of a battery. On the contrary, when the content of a conductive material is increased, the electroconductivity of an electrode and high-rate charge characteristics are improved but the adhesion between lithium metal oxide and a current collector is decreased, thereby making it difficult to realize desired performance.
Further, there is a disadvantage in that the diffusion rate of lithium ions in an active material is low. To solve the problem, LTO particles are prepared to have a small particle size less than 1 μm. However, in this case, LTO has an increased specific surface area and requires a large amount of binder, and has a difficulty in dispersion. Therefore, there have been suggested secondary particles formed by aggregation of primary particles. However, the pore size and distribution in the particles are not uniform, thereby causing excess or deficiency of an electrolyte and non-uniformity in availability of an active material.
Under these circumstances, there is a need for a negative electrode material which overcomes the disadvantages of a lithium titanium oxide and has low internal resistance, high electroconductivity and excellent output characteristics.